Two soldiers, both reportedly American, were killed in the east of Afghanistan, as the Taliban called on Afghans to attack foreigners and President Barack Obama made a formal apology over the reported burning of Qurans at a US air base.

Afghan demonstrators run as they shout anti-US slogans during a protest against Quran desecration in Kabul on February 23, 2012. The Taliban exhorted Afghans on February 23 to attack and kill foreign troops to avenge the burning of Qurans at a US-run base, but stopped short of cutting off contacts with American officials in Qatar. SHAH MARAI/AFP/Getty Images (Shah Marai/AFP/Getty Images)

Two United States troops have been killed by an Afghan soldier angry about the burning of Qurans in Kabul, CBS News reported.

The news came as President Barack Obama sent a formal letter of apology to his Afghan counterpart, Hamid Karzai, over the incident at Bagram Airbase in which Qurans were disposed of in a waste incinerator.

According to CBS, the International Security Assistance Force (ISAF), the international coalition in Afghanistan, has confirmed only that two troops were killed in eastern Afghanistan on Thursday by "an individual wearing an Afghan National Army uniform." An unnamed Afghan official told the network that the two soldiers were Americans, and that the attack appeared to be motivated by the Bagram incident.

"The error was inadvertent; I assure you that we will take the appropriate steps to avoid any recurrence, to include holding accountable those responsible."

Karzai last night called on protesters and security forces alike to show restraint and "strictly avoid resorting violence."

At least six more Afghans died in a third day of protests today, the BBC reported. Two people were shot in Khogyani in Nangarhar province – where the two ISAF soldiers were killed – when armed men reportedly attacked a military base and NATO forces returned fire. Another two were killed on clashes with Afghan security forces in southern Uruzgan province, one in Baghlan province in the north, and one in Laghman province east of Kabul. Dozens of others, including police, were reported injured.

Witnesses described crowds chanting "Death to Obama" and burning US flags.

The New York Times reported that the Taliban had exhorted people to take up arms against foreign troops and the Afghan security forces:

"In a message to the media and posted on Taliban web sites, the insurgent group gave specific instructions including to 'attack the occupiers' military bases, their military convoys and other occupying soldiers.'"

The US embassy in Kabul remains on lockdown, according to Bloomberg. In an email, the embassy said it had "suspended all travel for embassy personnel in Kabul until further notice and ordered any personnel not at a secure compound to return to the embassy immediately."

It tweeted:

Peaceable assembly is an American value/tradition; we join President Karzai in urging restraint and nonviolence today.

Burning a Quran is a deeply sacrilegious act in the Islamic faith. Sultan Shahin, an Indian analyst of Muslim and interfaith affairs, told Bloomberg: "Americans often suppose that the Koran, as a book, is analogous to the Christian Bible, but in fact Muslims' reverence for the Koran is more like the reverence that Christians feel for the person of Jesus."